1. Field of the Invention
The present invention relates to a method of pre-heating a pot provided with anodes and cathodes for the production of aluminium by electrolysis.
2. Description of the Related Art
Aluminium is produced industrially by igneous electrolysis, in other words by electrolysis of the alumina in solution in a molten cryolite bath. This bath is contained in a pot including a steel shell, which is coated internally with refractory and/or insulating materials, and a cathode assembly located at the bottom of the pot. Anodes of carbonaceous material are partially immersed in the electrolysis bath. The electrolysis current, which flows in the electrolysis bath and the pad of liquid aluminium via the anodes and the cathode elements, implements the reactions that reduce the alumina and also allows the electrolysis bath to be kept at a temperature of about 950° C.
The pots are arranged in series and are subjected to a current of the same intensity.
However, before the aluminium itself can be produced, it is necessary to warm up the pot, which is initially cold. This is a delicate operation during which thermal shocks need to be avoided. In fact, a pot demands very substantial investment and has a life cycle typically of between 3 and 7 years. It is therefore necessary to take every precaution so as not to reduce the pot's period of service. To this end, the rise in temperature within the pot must be slow, typically of 20° C. per hour.
In a known method of pre-heating, a uniform layer of a granular conductive material is deposited between the anodes and the cathodes, this layer then allowing a method of resistance pre-heating of the pot.
A proposal has already been made to use a carbonaceous material and more particularly coke as the granular conductive material. Using coke produces too high a resistance making it essential to use shunts which are progressively removed (as described in “Cathodes in Aluminium Electrolysis”, by M. Sørlie and H. A. Øye, Aluminium Verlag, 1984, pp. 77-83).